SHORT STROKE MOTOR Build

[spannerhead]

I'm considering building a short stroke. I know this has been discussed before but as I'm sort of new here have a lot to learn.

I have a 2.4 MFI or a 2.7 CSI to work with. I want a street come track engine. I have been reading as much a I can but as always get confused by all the various options and opinions.

If one where to have it built here in Aus. Who has the experience ? Any recommendations ?

I would like to build it my self but would have to farm out for all the machining ect. Have any of you done this ? What was your experience ?

Appreciate any and all comments . Thank you.

[Fishcop]

Much peril in building anything 'custom' yourself... A lot of people buy up all the bits they think they need only to find there are many incompatibilities in the chosen parts. Most machinists do not know Porsche engines and will go exactly off your instructions - any errors are your (expensive) problem to resolve.

I've built myself my own engines with much success, however I've stuck to factory build sheets and not strayed too far off known combos. Having said that, my current project is getting a 2.0 S spec engine built for it by Michael Harris at Promotive in Brisbane as I'm time poor and he is seriously experienced and does most work in-house without farming out.

Why the desire for a SS engine? What is it's intended purpose? SS engines nearly always have to be re-geared to take advantage of a different power band. Factor that in...

Personally I'd avoid buggering up a 2.4 MFI engine. The 2.4 is desirable because that case is the 7R (strongest of the mag cases) and the only one with "small piston spigots", it's a rare case and MFI combo that is worth more to you intact.

The 2.7 CIS uses the same 7R case but the spigots are larger to accommodate the 2.7 barrels and that engine already has better heads. Here's where you can have fun as you have the base of a 2.7RS spec motor with a potential to squeeze 230hp using carbs, bigger cams and higher compression. Bang for buck, this engine combo or a derivative is relatively easy to build and wonderful in an early car.

If you must have a SS engine, I'd really advise you to talk to the well known Porsche builders for advice. Nearly everything will need to be custom made based on the engines you have. Nothing impossible, but I'd guess your machining costs alone are going to run into $3-5k and there will be much trial and error to get it right - build it up/check/strip it down/repeat/replace.

There are a heap of engine build threads on the Pelican Parts website that may help you out to decide (my own 2.7RS build is there somewhere). Lots of mistakes documented on there too

[CamBiscuit]

My old orange car had a fairly spanky 2.3 twin plug engine ......

What capacity are you thinking? What gearbox are you pairing it with?

There's a huge amount of variables here.

[epilot]

once you start moving away from the standard spec on your MFI the costs escalate very quickly. MFI Werks are about $1600 to service the pump, plus about $500 for a new space cam, to modify your stacks and TB's is easily another $2300, all USD, cams $1000, P's and C's $4 - 6K depending on your spec

[DJ911]

2.7RS spec motor with a potential to squeeze 230hp using carbs, bigger cams and higher compression.

Sorry, off topic but curious, my car has been dynoed at 140kw (187bhp) at the rear wheels. It's a 3.0 running a 915 box, any idea what drive train power loss is likely to be?

[Fishcop]

Sorry, off topic but curious, my car has been dynoed at 140kw (187bhp) at the rear wheels. It's a 3.0 running a 915 box, any idea what drive train power loss is likely to be?

I think they work on a 15% drive line loss, so you'd be well above 200hp off the fly wheel. My 2.7 is standard "RS" with the factory 8.5:1 CR and does about 200hp at the flywheel. The correct MFI would probably give me another 7-10hp.

[DJ911]

I think they work on a 15% drive line loss, so you'd be well above 200hp off the fly wheel. My 2.7 is standard "RS" with the factory 8.5:1 CR and does about 200hp at the flywheel. The correct MFI would probably give me another 7-10hp.

Thanks John.

[Lyndon]

I wouldn't count on 15% loss. I know it makes a dyno sheet look nice, however if you had a 15-20kw heat pump in the transmission you would have nothing but melted gear teeth. Just saying.

[Fishcop]

Yes I agree the percentage is pretty subjective. I saved some stuff a while ago which discusses it all (can't seem to find). And let's not start on the variables when using different types/brands of dyno

[Cairns888]

I had an early 2.0 litre alloy case with 2.8 RSR P and C's. 66 mm crank. S cams. 2.4 S heads.
It was a ripper. Revved hard and fast. Approx 2.6 litre capacity.
Ran out of gears at 200 kph ish. Not sure if it would suit a taller box or not. It was a very light car.

All depends on what you want it for.
So many options as everyone has mentioned.
You could always get a shorter box and that will give you a quicker car but will rev higher and louder on the highway.
There was a good list in a Porsche engine book I read. Popular book but I can't remember the name of it.
Find out what you want from the car and then build the engine to suit.
Then work out if you are comfortable on he road with it or if you want it for track days.

If you want to sell the 2.4 MFi let me know. I think that suits my 71T...I think.

[spannerhead]

I thank you all for your comments and input. Yes there are many ways to go. As you say I really need to narrow down the motors requirements. I would like to do some hill climb's witch was why the SS appealed , but keeping it steerable as well ! A tall order.

Still confused as to witch way to go in terms of spec. The 2.7 seems the better option.

[Fishcop]

If you really want to go the whole 9 yards, the same 7R case/crank can be built to 2.8 litres and 11.5:1 CR (RSR spec) - around 240hp but questionable long term reliability

[PCM911T]

Dear Mr Spannerhead, make up your mind, you are driving me crazy! Ok I feel better now. In the meantime, you might like to read Wayne Dempsey's handbook How to Rebuild and Modify Porsche 911 Engines 1965-1989 as it has details on just about all the modifications possible.
My humble advice though is to use the engine that best date matches the year of your car. In standard form, unless you are a driving ace, the standard engine will give you plenty of fun on the road and for hill climbs. Learn to drive it first and then go for more power if necessary. A matching engine will make the car more valuable and cause you less insurance hassles.
Cheers
Paul M

[spannerhead]

Dear Mr Spannerhead, make up your mind, you are driving me crazy! Ok I feel better now. In the meantime, you might like to read Wayne Dempsey's handbook How to Rebuild and Modify Porsche 911 Engines 1965-1989 as it has details on just about all the modifications possible.
My humble advice though is to use the engine that best date matches the year of your car. In standard form, unless you are a driving ace, the standard engine will give you plenty of fun on the road and for hill climbs. Learn to drive it first and then go for more power if necessary. A matching engine will make the car more valuable and cause you less insurance hassles.
Cheers
Paul M

Hello Paul

Well sorry to drive you crazy ! I have looked at Dempsey's book witch opened up all sorts of options, hence my interest in what the consensus was.

By the way how do you know weather I can drive or not ? I grew up in the back of my fathers 72 RS driving to many different tracks where he raced many types of cars. So its kind of in my blood.

[9er]

I don't think Paul was questioning your driving ability.
If it is any help my engine build has gone in the opposite (well not exactly) direction where I have sought extra capacity from oversized pistons and also fitting the later longer stroke crank. I think I am ending up at about 2.55 litres.
I am trying to get about 180 easy driving torquey horsepowers in about 1000KGs